A sectional view of a conventional twisted nematic (TN) liquid crystal display is schematically shown in FIG. 3. In order that the nematic liquid crystal molecules are aligned to form a twisted spiral construction, a diagonal evaporation method or a rubbing method may be employed. The rubbing method is a method wherein a surface of an electrode substrate to be contacted with a liquid crystal is rubbed with cloth in one direction. The two electrode substrates 1 and 2 thus rubbing-treated are positioned to face each other such that their aligning directions are at right angles to each other and a nematic liquid crystal 7 having a positive dielectric anisotropy is put therebetween. The liquid crystal molecules have a spiral construction twisted at 90.degree. between the electrode substrates 1 and 2. In FIG. 4, 10 indicates the rubbing direction of the electrode substrate 1 and 11 indicates the rubbing direction of the electrode substrate 2. Polarizing plates 8 and 9 are respectively placed on the electrode substrates 1 and 2, wherein a polarizing direction 12 of the polarizing plate 8 approximately equals the rubbing direction 10 of the electrode substrate 1 and a polarizing direction 13 of the polarizing plate 9 approximately equals the rubbing direction of the electrode substrate 2 (FIG. 4). In FIG. 3, 3 and 4 indicate transparent electrodes and 5 and 6 indicate alignment layers.
On the other hand, when such a conventional matrix type liquid crystal display is driven by an optimum voltage averaging method of frequency N, a ratio .alpha. of an effective voltage [Vrms(ON)/] at a lighting picture element and an effective voltage [Vrms(OFF)] at a non-lighting picture element is shown as follow: ##EQU1## In the above formula, when N.fwdarw..varies., .alpha..fwdarw.1. In other words, the larger the number N, the smaller the difference between Vrms (OFF) and Vrms (ON). Accordingly, in order to obtain a matrix type liquid crystal display having increased display information capacity, it is required for a liquid crystal display to operate with contrast even if the voltage difference between Vrms (ON) and Vrms (OFF) is small. It is therefore desired to develope a liquid crystal having sharp threshold properties or to improve an alignment film. Recently, increased display information capacity has been required in the field of the liquid crystal and the conventional twisted nematic liquid crystal display does not meet this requirement.
In order to obviate the above defects, it is proposed that a twist angle .phi. be adjusted to more than 90.degree. to improve multiplex driving properties. This is accomplished by adding an optically active agent in such an amount that a ratio (d/p) of a spiral pitch p and a liquid crystal layer thickness (d) nearly equals .phi./360.degree..
The inventors of the present invention have studied the relation of twist angle of a liquid crystal and contrast ratio and the result is shown in FIG. 6. It is apparent from FIG. 6 that the larger the twist angle, the more the contrast ratio. It also has been found that this tendency is the same for another liquid crystal.
However, in the case where the twist angle of a liquid crystal is more than 210.degree., if d/p is ore than a specific value, the alignment of the liquid crystal molecule, when applying a voltage, is disordered causing stripe domains to occur, thus light is scattered resulting in inadequate contrast ratio. The stripe domain occurs more frequently as the twist angle increases. The maximum d/p value above which the stripe domain does not occur becomes lower (see FIG. 7). Even a liquid crystal display having a large contrast ratio is difficult for actual use in view of reliability or uneven cell thickness when the width of d/p, i.e. .DELTA.d/p (.phi.), is less than 0.1.
For preventing the occurrence of domain, an aligning film is prepared by a diagonal evaporation method and the angle of a liquid crystal molecule to an electrode substrate, i.e. the pretilt angle, is enlarged to about 30.degree.. The diagonal evaporation method is difficult to use to form a uniform aligning film in a broad area.
The aligning film is presently formed by the rubbing method. In this method, the striped domain occurs and an improved liquid crystal is desired to be selected.